1. Field of the Invention
The present invention relates to a magnetic recording unit. More particularly, it relates to a magnetic recording unit adapted to carry out the tracking servo of a recording and reproducing head using a servo pattern recorded on a disk.
2. Description of Related Art
In conventional magnetic recording units having a magnetic head for recording and reproducing information, a servo pattern is formed on a magnetic disk for positioning the magnetic head thereon.
Conventional magnetic disks have both data regions for recording user's data and servo regions in which signals are recorded in advance for positioning the magnetic head of the magnetic recording unit. About several tens of servo regions extend radially on the magnetic disk from the center thereof at predetermined intervals.
The servo region is generally composed of a regular pattern used for automatic gain control (AGC), an index pattern indicating the beginning of a rotation, a Gray code pattern indicating a track number, and a servo pattern (also referred to as a position signal pattern) for obtaining a track positional information.
The servo pattern comprises patterns required for positioning the magnetic head of the magnetic recording unit. The patterns have, for example, a plurality of servo bits arranged in a zigzag relationship on the basis of a predetermined number of tracks. The servo bits are each read by the magnetic head to generate reproduction signals, and the amplitudes of the reproduction signals are then compared to obtain information as to where on a track the magnetic head is positioned in a width direction of the track, for adjusting the magnetic head, i.e., carrying out the tracking (Japanese Examined Patent Publication No. 47(1972)-32012).
FIGS. 15(a) and (b) are views for schematically explaining a conventional “amplitude detecting servo” system. FIG. 15(a) is a view illustrating the relationship between tracks and a servo pattern. FIG. 15(b) is a view illustrating examples of reproduction signals obtained by the magnetic head traveling on the servo pattern. The magnetic head 10 having a track width length Twr is to be positioned on a track #N. The magnetic head 10 travels in a direction of X shown in FIG. 15(a) to pass through patterns P and A–D for generating the reproduction signals as shown in FIG. 15(b). Black and white portions of each of the patterns P and A–D indicate magnetic directions of the servo pattern recorded on a magnetic recording medium that are opposite to each other. In other words, if the magnetic recording medium is an in-plane recording medium, the black and white portions of the servo pattern indicate vectors opposite to each other that have in-plane components in the track direction X, and if the magnetic recording medium is a perpendicularly recording medium, the black and white portions of the servo pattern indicate vectors opposite to each other that have components perpendicular to a surface of the magnetic recording medium. These patterns are shown schematically in FIG. 15(a) and the actual signal periods thereof are shown in FIG. 15(b).
When a difference between the amplitude SA of a reproduction signal of a pattern A and the amplitude SB of a reproduction signal of a pattern B is calculated, i.e., a subtraction SA–SB is done, and the magnetic head 10 is moved in a track width direction y, the result is a signal N-POS shown on the right of FIG. 15(a). Likewise, when the difference between the amplitude SC of a reproduction signal of a pattern C and the amplitude SD of a reproduction signal of a pattern D is calculated, i.e., a subtraction SC–SD is done, and the magnetic head 10 is moved in the track width direction y, the result is a signal Q-POS shown on the right of FIG. 15(a). Then, by properly utilizing the above signals N-POS and Q-POS as position signals, the current position of the magnetic head 10 can be grasped.
A “phase detecting servo” system, another conventional servo system, is disclosed in Japanese Unexamined Patent Publication No. 60(1985)-10472). FIGS. 16(a) and (b) are views for schematically explaining the “phase detecting servo” system. The magnetic head 10 having with the track width Twr is to be positioned on the track #N. The magnetic head 10 travels in the direction of X shown in FIG. 16(a) to pass through the patterns P and A–C for generating reproduction signals as shown in FIG. 16(b). The black and white portions of each of the patterns P and A–C have the same meanings as in FIG. 15(b). The patterns have an azimuth angle of tilt for the magnetic head 10, though the angle is too small to cause a problem of degradation (azimuth loss) in the reproduction signals and therefore, the waveform of each reproduction signal is regarded as being almost the same as that in FIG. 15(b). The phase of each of the patterns A, B and C from the pattern P, however, varies depending on where in the track width direction (y) the phase is measured. Here, the phases of the patterns A, B and C on the track #N are given as PA, PB and PC, respectively. These patterns are shown schematically in FIG. 16(a) and the actual signal periods thereof are shown in FIG. 16(b).
When the respective phase differences PB–PA and PC–PB are calculated, an example of the result in the y direction is shown on the right of FIG. 16(a). By properly utilizing the above signals PB–PA and PC–PB as position signals, the current position of the magnetic head 10 can be grasped. Japanese Unexamined Patent Publication No. Hei 9(1997)-312073, for example, discloses a method for obtaining the phases PA, PB and PC from reproduction signals of FIG. 16(b). In a pattern of this phase detecting servo system, one period is composed of a large number of tracks, so that code information indicating the position of the magnetic head on the disk is reduced in amount in comparison with that of a pattern of the amplitude detecting system.
Today, a demand has been made for further increasing the recording density by decreasing the unit data bit intended for storage or by lowering a ratio of the servo region to the disk even slightly.
In conventional servo detecting systems as mentioned above, however, it is difficult to increase the recording density while maintaining the reliability in recording and reproducing.
Conventionally, information in the servo region is recorded using a servo track writer (STW). The servo track writer, however, requires a long time for recording information since it employs magnetic head. In order to solve this problem, there has been proposed a servo information transfer method for transferring the servo pattern information and the like recorded on a master disk to a slave disk all at once using a magnetic transfer technique in a shorter time than conventionally.
In this method, however, it is difficult to carry out a uniform transfer when a master disk and a slave disk are not in sufficient contact because of the presence of dust or the like. In a non-uniform transfer, when the servo pattern is reproduced with the magnetic head, reproduction signals varying in amplitude are generated, resulting in insufficient positional reliability. This problem becomes more outstanding as the density of the servo pattern is increased.
Also, when a misalignment occurs between the slave disk and the master disk, an eccentric slave disk may possibly be produced.